1. Field of the Invention
The present invention relates to a grism, and more particularly to a grism used suitably for monitoring and/or inspections apparatuses in a factory and the like for scientific observation.
It is to be noted herein that xe2x80x9cgrismxe2x80x9d is a transmission type direct vision dispersive element prepared by combining prisms with a grating so as to allow a light beam having an arbitrary order and an arbitrary wavelength to go straight.
2. Description of the Related Art
In recent years, a number of instruments for astronomical observation having both functions of imaging and spectroscopic function have been developed with progress of two-dimensional detectors. In such an instrument, grisms are used for a dispersive element.
In this case, a so-called high dispersive grism provided with a replica grating has been proposed as a diffraction grating. In such grism, since a refractive index of a resin being a material for preparing a replica grating is around 1.5, there has been such a problem that high resolving power cannot be attained because a light beam exceeds its critical angle in the case where a prism to which is attached the replica grating has a refractive index of, for example, around 2.3, even if the vertex angle of the prism is around 40xc2x0.
Furthermore, the above-described high dispersive grism provided with a replica grating may be used within a range where a light beam does not exceed the critical angle, as a matter of course, but there has been a problem of disadvantage in efficiency.
In this respect, if a diffraction grating can be directly processed and formed on a surface of a prism, high efficiency can be achieved. However, there has been such a problem that it is difficult to directly process a diffraction grating having a depth of 1 xcexcm or more on a surface of a prism. Moreover, mass-production is difficult, whereby a grism produced by means of a direct process becomes expensive.
3. Object and Summary of the Invention
The present invention has been made in view of the above-described problems involved in the prior art, and an object of the invention is to provide a grism wherein a light beam does not exceed its critical angle even if the vertex angle of a prism is made to increase, its efficiency can be elevated, besides, mass-production thereof can be made, and a low cost therefor can be realized.
In order to achieve the above-described object, a grism according to the present invention is constituted by combining prisms each prepared from a material having a high refractive index with a volume phase grating (Volume Phase Grating: VPG).
Namely, a grism according to the present invention comprises a first prism having a high refractive index, a second prism having a high refractive index, and a volume phase grating used for a diffraction grating; the vertex angle of the above-described first prism being opposed to the vertex angle of the above-described second prism so as to sandwich the above-described volume phase grating between the first prism and the second prism; a light beam being input from the outside through a surface of the above-described first prism; the light beam input inside the first prism being input into the above-described second prism through the above-described volume phase grating; and the light beam input a inside the second prism being output to the outside through a surface of the second prism.
Furthermore, the grism according to the present invention is characterized in that a refractive index of the above-described first and second prism are higher than a refractive index of the volume phase grating.
Moreover, the grism according to the present invention is characterized in that a material for preparing the above-described first prism is either of zinc sulfide and lithium niobate; a material for preparing the above-described second prism is either of zinc sulfide and lithium niobate; and a material for preparing the above-described volume phase grating is bicromate gelatin.
Besides, the grism according to the present invention is characterized in that a sum of the vertex angle of the above-described first prism and the vertex angle of the above-described second prism is equal to or larger than a critical angle determined by a refractive index of the first prism and a refractive index of the volume phase grating.